By Clint
When I was young, my interest in guns was as any other child’s was. Namely an interest in the idea that guns made you strong. As I got older, I grew out of that, they weren’t strength, they were tools, and real strength was determined by other factors. At a time in my life where a young person’s interests are shaped, I got into video games. Guns weren’t interesting, cool, but not a fascinating topic, but one game I played changed that in an odd way.
Tom Clancy’s Rainbow Six got me interested in the topic of firearms. Though it wasn’t the idea of playing a super secret squirrel agent of an oddly named international anti-terrorist group that got me hooked. What caught my interest was a decision the player must make when gearing a character. FMJ or JHP? As a kid my response was “I dunno” but I was a curious kid, and I wanted to know. Which is better, and which should be used and when?
That’s what got me into guns. As I learned more, I came to research ammunition, and later, guns themselves. All while learning to brush aside what I call “Projectile Vomit”. Basically, this covers any comments being spewed by a person with a near religious fervor for a specific caliber, brand, or type of projectile, or an equally exceptional lack of understanding of ballistics, and no real information to back up the claims.
“A .45 caliber will kill the soul” “.40s suck” (this one’s actually probably true, but the hypothesis still qualifies) “FMJs are like ice picks, in the body, while Hollowpoints are like bombs” “.45 because shooting twice is silly” These are all examples of projectile vomit.
Now that I’m older, and looking into ammunition for a more hands-on version of the hobby, the projectile vomit is getting more and more sophisticated. Usually involving Dr. Sydney Vail, FACS vs. Dr. Gary Roberts, DDS, or professionalsoldiers.com vs. tacticalforums.com. Or nearly any sentence that begins with “The FBI and IWBA.” The biggest instances of projectile vomit I’ve noticed in the area of ballistic research involves sectional density and momentum.
Sectional density is one I had to look up. I was familiar with tensile strength and elasticity and stress/pressure, but sectional density was a new one for me. So I looked it up, and hoo-boy did Wikipedia fail. The line “SD_Ballistics = Mass/Diameter^2 (approximately equal to) pressure” immediately showed me what I needed. (Funnier even, right after saying approximately equal to pressure, they got the derivation for the Pascal wrong).
The only instance sectional density would be approximately equal to pressure is if every bullet of a given caliber moved at the same velocity as every other bullet of the same caliber regardless of mass.
To explain, pressure is force applied to a given area ex. kg/(ms^2) (the Pascal) or lbf/in^2 (or PSI). Note that the description of the PSI is not a typo. PSI is not pounds (weight) per square inch; rather it’s pound-force per square inch. Both of these consider force, which is a function of mass and velocity (specifically the change in velocity over the change in time, aka. acceleration), whereas sectional density only considers mass. Why is this important? Simple, bullets fired from different cartridges have, both, different mass and different velocity.
This also transitions to the second mentioned issue of momentum. Momentum is the velocity of an object modified by the object’s mass and vice versa. Momentum doesn’t grant any special powers; rather it’s just an observation of two attributes of an object at a given moment in time. If you increase mass of an object by 63.33%, you have to retain at least 61.23% of the velocity, to get the same momentum (1.6333 * .6123 = 1.0001). For reference 63.33% is how much of an increase in mass you get, when going from a 90gr projectile to a 147gr. This means that you only increase the momentum from a 9mm, when you go to 147gr from 90gr, if your 147gr velocity is at least 61.23% of the 90gr projectile’s velocity.
Pop quiz: which has more momentum, a 77gr. SMK .223 at 2750fps or a 147gr. 9mm HST at 1050fps? Would you believe someone who told you that simply because of the increased mass, the HST has more momentum?
The projectile vomit comes when people claim that because of sectional density and momentum, the heavier the bullet, the greater the penetration. In other words, when the claim is made that light and fast is, due to physics, not capable of performing as well as slow and heavy, regarding penetration and reaching vitals you have projectile vomit. This quite simply can’t be true.
Because more momentum means it will take more force to slow an object, in a given timeframe, and both mass and velocity are factors, it shouldn’t matter which get’s increased vs. decreased as long as the correct proportionate change is maintained. Additionally as stress is the determinant for overcoming tensile strength, not sectional density, a lighter and faster bullet that has more energy to apply more force to the same area, will then apply more stress to an object, and obviously penetrate better. As stress is force per area not mass per area.
Does that mean that light and fast is the way to go? Maybe not. There are a multitude of factors to terminal ballistics, and obviously heavy/fast would be the ideal. A heavier projectile moving at velocities closer a light/fast round, would have more energy for use in the same area, more momentum, and more material to work with when expanding/fragmenting, but that’s not sectional density and that’s not momentum ignoring velocity’s role. What this does show is a level of projectile vomit that should make you take a step back and not necessarily take everything an institution or expert says as ballistic gospel, and instead look and rounds and their given attributes against others, without preconceptions and pre-decisions.
So, two questions for readers: First, what is the worst instance of projectile vomit you’ve ever heard? Bonus points if it’s this article and you can show me I’m wrong. I do love it when that happens. Second, do you enjoy researching the ammunition and what makes a round effective as much as looking into guns themselves?